Plot a horizontal line using matplotlib

Question

I have used spline interpolation to smooth a time series and would also like to add a horizontal line to the plot. But there seems to be an issue that is out of my grips. An

Answer 1

In addition to the most upvoted answer here, one can also chain axhline after calling plot on a pandas's DataFrame.

import pandas as pd

(pd.DataFrame([1, 2, 3])
   .plot(kind='bar', color='orange')
   .axhline(y=1.5));